In suspension systems for tractor-trailer combinations, it is common to locate the roll center of the vehicle above the elevation of the vehicle axles. Such an arrangement will place the center of gravity of the trailer load relatively close to the roll center, thereby reducing the size of the roll moment caused by the lateral acceleration of the payload relative to the roll center of the suspension.
This arrangement is not ideal when a vehicle axle encounters an upward or downward force on one side of the axle. Because the axle is constrained to move about the roll center, an upward or downward force on one side of the axle will cause the axle to rotate about the roll center. Such rotation will require a lateral shift between the axle and the vehicle chassis. FIG. 1 illustrates a vehicle axle A having left and right wheels B, C attached to either side thereof and resting on a roadway surface S. The roll center R is positioned above the vehicle axle, as per common prior art suspension systems. When the vehicle turns, the center of gravity CG of the payload will be displaced relative to the roll center R, caused by the lateral acceleration. This lateral acceleration is counteracted by a force in the lateral “torque rod” D (panhard rod), which together cause a destabilizing moment. An “anti roll bar” is employed to counteract this moment and reduce the lateral displacement of the CG. This will reduce the total vehicle destabilizing moment due to the weight of the vehicle acting at a position that is laterally offset from the center of the wheel track.
U.S. Pat. No. 4,667,974 discloses a vehicle suspension system having a low roll center to reduce lateral shifting and jolting, in combination with a stabilizer assembly to provide high roll stability. In a preferred embodiment, the roll center of the vehicle is positioned below the vehicle axles to reduce lateral shifting of the wheels with respect to the chassis. The stabilizer assembly is positioned between the longitudinal beams below the axles and couples each axle of the tandem axle pair to the other to require counterbalancing movement of the axles for high roll resistance. The stabilizer assembly is mounted to the vehicle chassis at locations spaced apart from each axle to further reduce the degree of lateral shifting of the wheels with respect to the chassis. A problem with this solution is that the lateral displacement of the CG becomes larger for the same lateral acceleration, which must be compensated with a larger stabilizer, thus compromising ride quality.
Background art also includes axle suspensions that may comprise an axle guiding means using longitudinal control arms, which are arranged on both sides of the vehicle and are articulated to the vehicle axle at the one end, and to the vehicle body, or chassis at the other end. According to one example, described in U.S. Pat. No. 5,711,544, such suspensions may be provided with a triangle pull rod anchored in the center of the vehicle with a central joint, preferably on the vehicle axle at one end, and is articulated on the vehicle body with the ends of its two struts at the other end. A stabilizer bar is arranged directly between said longitudinal control arms, underneath said pull rod and the frame of the vehicle.
A problem with this solution is that it is not possible to lower the vehicle without the stabilizer assembly entering the cargo space.
Alternative solutions may comprise U-shaped stabilizer bars combined with lateral torque rods.
One problem with the described suspensions is that they may contribute to roll steer after the vehicle has rolled, subsequent to a lateral load being applied to the vehicle. The solution according to the invention will instead contribute to understeer, which is a desired steering response under lateral load. A further problem with U-shaped stabilizer bars combined with lateral torque rods is that it may “bind” at cross articulation or roll, that is, overstressing of the stabilizer bar or adjacent components may occur.